Water Cooling Flow Rate and Heat Transfer

NOTE: This was written by one of our Forum Members – I thought it and the discussion that follows is worth reading.

There is an elementary equation from basic thermodynamics that states that the rate of heat transfer (Q) equals the mass flow rate (M) times a Constant (the specific heat of water) times the Delta T (fluid temp out minus fluid temp in):

Q = M x C x Delta T

In other words, the rate of heat transfer is directly proportional to mass flow rate. If you increase the flow rate, you will then increase the rate of heat transfer. Since you cannot mess with mother nature, it is very naive to think it works any other way.

Assume the CPU inserts a constant rate of energy (Q) into the cooling system. Then, from the relationship above, increasing the mass flow rate must result in a smaller delta T because Q remains constant. This smaller Delta T (fluid out – fluid in) also means that the average fluid temperature in the water block is somewhat lower even though the rate of heat transfer has not changed.

Now let’s look at the heat transfer from the CPU to the water:

The rate of heat transfer between two points is proportional to the temperature difference between those points.

In our case, this Delta T (not to be confused with the one above) is the temperature of the CPU minus the average water temperature in the water block. Lowering the average water temperature, as we did above by increasing the flow rate, means we have a little better heat transfer from the CPU to the now somewhat cooler water. The result is that the CPU runs a little cooler.

This all says that if you increase the flow rate and everything else remains constant, you will decrease the CPU temperature. However, everything else will not remain constant if you increase the flow rate by using a larger pump.

The pump uses some amount of electrical energy. This energy must end up somewhere. A relatively small amount of it is dissipated as heat from the motor. The overwhelming majority of it is converted from electrical energy to mechanical energy in the form of a rotating shaft that does real work on the water.

This energy ends up in the water by increasing its temperature. It is called “pump heat” and can be very significant.

An Eheim 1048 is rated at 10 watts, almost all of which ends up in the water. I understand a very overclocked CPU is good for upwards of 75 watts. As you can see, a smaller pump like the 1048 contributes about 13% to the total heat load on a system with an energy hungry CPU. With other more common CPUs running at 25 to 50 watts, this percentage is much higher and is therefore much more significant.

An interesting aside for non-believers: This is also why excessive use of a blender to mix up frozen orange juice results in the juice not being as cold as expected. Also, nuclear power plants use primarily pump heat (from three or four 6,000 HP pumps) to heat up almost 75,000 gallons of water from 200 degrees F to about 550 degrees in about six hours or less.

The point here is that there is a trade off in how big a pump to use to increase the flow rate. More flow is beneficial. It is best to achieve the desired flow with as small a pump as possible and flow paths with minimum flow resistance.

The bigger the pump, the more heat is added to the system. Eheim makes a 50 watt unit that I see talked about every now and then. This guy is probably a bigger heat load on the cooling system than the CPU itself.

Bottom Line

If you increase flow rate with the same pump, your temperatures will trend in the direction of goodness. If you increase flow rate by going to a bigger pump, you will reach a trade off somewhere where the pump starts putting too much energy into the system and temperatures will start increasing.